46 CHLOROPHYCEJ;. 



may be found between the cells in the two filaments, those in the 

 one whose protoplasmic contents pass over being cylindrical, while 

 those of the recipient one are more barrel-shaped, and of a larger 

 diameter. The former may be regarded as a male, the latter as 

 a female plant. The zygote germinates after a period of rest, and 

 grows out into a new filament (Fig. 42). 



Order 3. Mesocarpaceae. The cell-walls are glabrous, uncon- 

 stricted in the centre, and united into simple unbranched filaments. 

 The chromatophore consists of an axial chlorophyll-plate, with 

 several pyrenoids. The zygote is formed by the coalescence of two 

 cells (Fig. 43) (sometimes three or four), but the whole proto- 

 plasmic contents of the cells do not take part in this process, a 

 portion always remaining behind ; the aplanogametes coalesce in 

 the conjugation-canal. The zygote thus formed appears incapable 

 of germination until after 3-5 divisions. Of the cells so formed, 



FIG. 48. Mougeotia calcarea. Cells showing various modes of conjugation : at m tripar- 

 tition ; pg quadripartition ; s quinquipartition of the zygote. 



only one is fertile, the sterile cells, according to Pringsheim, con- 

 stituting a rudimentary sporocarp. The germinating cells grow 

 out into a new filament. In this order, conjugation has been ob- 

 served between two cells of the same filament. The Mesocarpacea? 

 thrive best in water which contains lime. 



Class 6. Chlorophyceae (Green Algae). 



These Algse are coloured green by chlorophyll, seldom in com- 

 bination with other colouring matter, and then especially with red. 

 The product of assimilation is frequently starch, which generally 

 accumulates round certain specially formed portions of protoplasm 

 termed pyrenoids. The thallus is uni- or multi-cellular; in the 

 higher forms (certain Siphonese) the organs of vegetation attain 

 differentiation into stem and leaf. The asexual reproduction takes 

 place in various ways ; the sexual reproduction is effected by con- 

 jugation of motile gametes, or by oogamous fertilisation. The 



